Abstrict A strobed blood flow meter provides periodic measurements of blood
flow velocity or volumetric blood flow over a cardiac cycle at reduced
average power consumption, which is advantageous for reducing battery
size, and extending device battery life, such as in an implantable
application. Continuous wave Doppler, pulsed Doppler, laser Doppler,
transit time, electromagnetic flow, and thermal dilution techniques
are included. Strobing provides higher level excitation during active
periods, which improves signal-to-noise ratio, and provides a low
power standby mode during an idle time between active periods. The
invention may be used for chronic or acute applications. Doppler
or other signals may be telemetered from an implanted portion of
the flow meter for further signal processing to extract velocity
or volumetric flow. Alternatively, such signal processing is also
implanted, such that the velocity signal can be telemetered to an
remote monitor.
Claims What is claimed is:
1. A method of repeatedly estimating fluid flow in a conduit, the
method comprising the steps of: (a) illuminating the conduit with
ultrasonic energy for an active period of a control signal sufficient
to obtain an estimate of fluid flow; (b) receiving an ultrasonic
energy signal using one or more measurement circuits in response
to illuminating the conduit with ultrasonic energy; (c) processing
the received ultrasonic energy signal using the one or more measurement
circuits to obtain a fluid flow estimate; (d) interrupting step
(a) for at least a portion of an idle period of the control signal;
and (e) reducing power to at least a portion of the one or more
measurement circuits during the portion of the idle period.
2. The method of claim 1 further comprising repeating steps (a)-(d)
over a period of time.
3. The method of claim 2 wherein repeating steps (a)-(d) includes
repeating steps (a)-(d) at a strobing frequency greater than approximately
50 Hz.
4. The method of claim 1 wherein the active period in step (a)
is longer than a stabilization time.
5. A method of repeatedly estimating fluid flow in a conduit, the
method comprising the steps of: (a) powering up electronic circuits
to receive and process a signal representative of fluid flow in
the conduit to produce a fluid flow estimate during an active period
of a control signal; (b) illuminating the conduit with ultrasonic
energy from at least two sources to produce first and second signals
during at least a portion of the active period; (c) receiving the
first and second signals containing time-shifted frequencies and
(d) reducing power to the electronic circuits during at least a
portion of an idle period of the control signal.
6. The method of claim 5 further comprising repeating steps (a)-(d)
over a period of time.
7. The method of claim 5 further comprising powering on an amplifier
during step (a).
8. The method of claim 5 wherein step (a) comprises powering on
a receiver.
9. The method of claim 5 wherein a strobing frequency, corresponding
to the inverse of a sum of the active and idle periods, is greater
than approximately 50 Hz.
10. The method of claim 5 wherein step (a) comprises powering
on a processing circuit.
Description FIELD OF THE INVENTION
This invention relates to estimation of fluid flow, and more particularly
to a chronic or acute measurement of blood flow in a blood vessel.
BACKGROUND
There are many applications in clinical and research medicine in
which measurement or estimation of volumetric blood flow within
a blood vessel is desirable. One method of making such measurements
uses ultrasonic Doppler techniques to measure blood flow velocity
and thereby estimate volumetric blood flow. Velocity of an object
is often measured using the Doppler effect Single frequency ultrasonic
energy is transmitted into an area of tissue containing the blood
flow to be measured. This insonification of the area is typically
referred to as illumination Resulting ultrasonic energy is reflected,
or backscattered, from the illuminated area. Energy reflected from
moving targets, such as fluid and blood cells, will be shifted in
frequency from the illuminating frequency according to the well-known
Doppler effect The Doppler shifted frequency provides a measure
of the blood flow velocity.
In clinical and research applications, it is often necessary to
study blood flow for an extended period of time. Thus, in ambulatory
living organisms, such as animal or human subjects, there is a need
in the art to provide a battery-powered ultrasonic Doppler blood
flow meter for measuring blood flow velocity for an extended period
of time, allowing a human or animal patient freedom of movement
during the study and minimizing the need for supervision by the
clinician. There is also a need in the art to provide a small, low-power
ultrasonic Doppler blood flow meter that is suitable for implantation
in a human or animal subject. There is a further need in the art
to provide an implantable ultrasonic Doppler blood flow meter that
maintains adequate signal-to-noise (SNR) ratio for accurate velocity
estimation.
SUMMARY
The present invention includes a method and apparatus for estimating
blood flow or blood flow velocity in a blood vessel over a period
of time. According to the method, at least part of the measurement
circuits used to estimate blood flow are automatically activated
only during the time an estimate is being obtained. At least part
of the measurement circuits are automatically deactivated during
the time an estimate is not being obtained These steps are performed
repeatedly to provide a sequence of blood flow estimates forming
a blood flow waveform indicative of blood flow. More than one estimate
is typically required to obtain a waveform representative of the
blood flow.
The steps of activating and deactivating at least part of the measurement
circuits is repeatedly performed sufficiently frequently, either
periodically or at irregular intervals, such that the blood flow
waveform substantially represents the variable blood flow. Power
to at least a portion of the measurement circuits is reduced or
interrupted while the measurement circuits are deactivated.
Measurement of blood flow can be obtained through various blood
flow measurement techniques, including: continuous wave (CW) Doppler
flow measurement, pulsed Doppler flow measurement, laser Doppler
flow measurement, transit time flow measurement, thermal dilution
flow measurement, electromagnetic flow measurement, or other suitable
flow measurement technique.
In several embodiments, a basebanded Doppler-shifted signal provides
the blood flow estimate. In other embodiments, a blood flow output
signal is derived from the basebanded Doppler-shifted signal and
provided as the blood flow estimate.
Thus, the present invention provides a strobed blood flow meter,
such as an implantable strobed ultrasonic Doppler blood flow meter,
having reduced average power consumption, which is advantageous
for reducing battery size, extending battery life, and improving
signal-to-noise ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, like numerals describe substantially similar components
throughout the several views.
FIG. 1 is a block diagram of one embodiment of the invention.
FIG. 2 is a block diagram illustrating one embodiment of the mixer
of FIG. 1 in more detail.
FIG. 3 is a block diagram illustrating one embodiment of the transducer
of FIG. 1 in more detail
FIG. 4 is a block diagram illustrating one embodiment of the control
circuit of FIG. 1 in more detail.
FIG. 5A is a graph illustrating generally voltage vs. time waveforms
for one embodiment in which the invention is operated.
FIG. 5B is a graph illustrating generally a velocity vs. time signal
in operation of the embodiment of FIG. 5A, but on a compressed time
scale with respect to the illustration of FIG. 5A
FIG. 6 is a block diagram illustrating one embodiment of the present
invention in which certain components are turned off during the
idle period.
FIG. 7 is a block diagram illustrating another embodiment of the
present invention in which certain components are turned off during
the idle period.
FIG. 8 is a block diagram illustrating a further embodiment of
the present invention in which certain components are turned off
during the idle period.
FIG. 9 is a block diagram illustrating in more detail the control
circuit of FIG. 8 in more detail.
FIG. 10 is a block diagram illustrating an embodiment of the present
invention including an impedance matching network.
FIG. 11 is a block diagram illustrating an embodiment of the present
invention including a signal processor.
FIG. 12 is a block diagram illustrating one embodiment of the signal
processor of FIG. 11 in more detail.
FIG. 13 is a graph generally comparing the strobed continuous wave
and pulse Doppler ultrasonic frequency waveforms.
FIG. 14 is a block diagram illustrating one embodiment of the present
invention using transit time techniques of blood flow velocity estimation.
FIG. 15 is an end view of the embodiment illustrated in FIG. 14.
DETAILED DESCRIPTION
In the following detailed description, reference is made to the
accompanying drawings which form a part hereof, and in which is
shown by way of illustration specific embodiments in which the invention
may be practiced. These embodiments are described in sufficient
detail to enable those skilled in the art to practice the invention,
and it is to be understood that the embodiments may be combined,
or that other embodiments may be utilize and that structural, logical
and electrical changes may be made without departing from the spirit
and scope of the present invention. The following detailed description
is, therefore, not to be taken in a limiting sense, and the scope
of the present invention is defined by the appended claims and their
equivalents.
The present invention provides a strobed blood flow meter useful
for chronic or acute estimates of blood flow or blood flow velocity
and having reduced average power consumption, which has advantages
that include reducing battery size and extending battery life. As
discussed and defined herein, estimating volumetric blood flow and
blood flow velocity are understood as interchangeable concepts,
since estimates of volumetric blood flow are obtained from estimates
of blood flow velocity by multiplying blood flow velocity with a
known constant cross-sectional area of a blood vessel. When the
cross-sectional area of the blood vessel is unknown, a signal proportional
to estimates of blood flow can still be provided from estimates
of blood flow velocity since the cross-sectional area of the blood
vessel is assumed to be relatively constant.
As used herein, the term "strobing" is defined as repeatedly
estimating blood flow velocity during a period of interest, as discussed
below. In a living organism having a circulatory system with a cardiac
cycle, which is defined as the period between successive heartbeats,
the period of interest for strobing may be one or more such cardiac
cycles. However, it is also desirable to repeatedly estimate blood
flow velocity over a period of interest when no cardiac cycle is
present. For example, certain embodiments of an artificial heart
pump may be implemented without the periodic pulsing associated
with a heartbeat. In such systems, it may still be desirable to
repeatedly estimate blood flow velocity over some other period of
interest.
As will be described in detail below, the present invention encompasses
strobing or automatically activating certain portions of the blood
flow meter during an active period in order to obtain an ultrasonic
Doppler blood flow velocity estimate, and later automatically deactivating
these portions of the blood flow meter during an idle time between
such estates. As a result, average power consumption is advantageously
reduced. Strobing according to the present invention includes a
wide variety of blood flow measurement techniques, including, but
not limited to: ultrasonic Doppler blood flow measurement, such
as both continuous wave (CW) and pulsed Doppler blood flow measurements;
transit time measurements; electromagnetic flow measurements; thermal
dilution measurements; and laser Doppler measurements, each of which
is described further below.
FIG. 1 is a block diagram illustrating one embodiment of the present
invention. In FIG. 1 strobed ultrasonic blood flow meter 100 is
capable of being implanted in a human or animal subject for measurement
of blood flow in blood vessel 105. Blood flow meter 100 comprises
oscillator 110 which is a sine or square wave oscillator operating
at a carrier frequency in an ultrasonic region of the frequency
spectrum, typically in the 5-20 MHz range, though other frequencies
are also possible. The ultrasonic sine or square wave output signal
of oscillator 110 at node 115 is referred to as a carrier signal.
The carrier signal frequency at node 115 is in the ultrasonic frequency
range, and is electrically coupled to a control circuit 120 at control
circuit oscillator input 125. Control circuit 120 produces at control
circuit output 130 a resulting electrical strobed ultrasonic-frequency
signal (shown as signal 145V in FIG. 5A) which is electrically coupled
to amplifier input 135 of power amplifier 140 through node 145.
In response, amplifier 140 produces a resulting electrical strobed
amplified ultrasonic-frequency signal at amplifier output 150 which
is electrically coupled through node 165 to transducer electrical
input 155 of transducer 160. In response, transducer 160 provides,
at transducer ultrasound output 170 ultrasonic energy that is mechanically
or acoustically coupled to tissue including blood vessel 105. In
this patent application, providing ultrasonic energy, insonifying,
and insonating, are all referred to generally as illuminating.
Illumination of blood vessel 105 results in a reflected Doppler-shifted
ultrasound signal, also referred to as a backscattered signal, that
is received at transducer ultrasound input 175 and converted by
transducer 160 into a Doppler-shifted electrical signal at transducer
electrical output 180. The Doppler-shifted electrical signal is
electrically coupled through node 195 to receiver input 185 of receiver
190 which provides a buffered Doppler-shifted signal in response
thereto at receiver output 200.
Mixer 205 receives the buffered Doppler-shifted signal at mixer
input 210 through node 215. Mixer 205 also receives through node
115 the carrier signal of oscillator 110 at mixer oscillator input
220. Mixer 205 performs a demodulation function by quadrature mixing,
as described below, producing an in-phase (I) signal at in-phase
(I) output 225 and a phase-shifted (Q) signal, which is 90 degrees
out of phase with respect to the I signal, at phase-shifted (Q)
output 230. The I and Q signals each have components that include
difference and sum frequency components that are approximately equal
to the respective difference and sum of the frequencies of the carrier
signal and the buffered Doppler-shifted signal. The I and Q signals
may also contain a carrier frequency component, also referred to
as carrier feedthrough.
The I signal is electrically coupled through node 235 to a first
low pass filter input 240 of first low pass filter 245. First low
pass filter 245 removes the carrier feedthrough and the sum frequency
components of the I signal, and provides the difference frequency
component at the first low pass filter output 250. The difference
frequency component at the first low pass filter output 250 is referred
to as the basebanded in-phase Doppler signal or the basebanded I
Doppler signal. Similarly, the Q signal is electrically coupled
through node 255 to a second low pass filter input 260 of second
low pass filter 265. Second low pass filter 265 removes the carrier
feedthrough and the sum frequency components of the Q signal and
provides the difference frequency component at the second low pass
filter output 270. The difference frequency component at the second
low pass filter output 250 is referred to as the basebanded phase-shifted
Doppler signal, or the basebanded Q Doppler signal.
The basebanded I and Q Doppler signals are electrically coupled
through respective nodes 275 and 280 to respective inputs of telemetry
circuit 285. In one embodiment, the basebanded I and Q Doppler signals
are remodulated with a telemetry carrier frequency for transmission
to a remote telemetry device 282 such as an external telemetry
receiver. In another embodiment, as described below, an analog velocity
output signal is produced, which is encoded, such as by pulse position
modulation, for transmission to remote telemetry device 282. Thus,
telemetry circuit 285 allows transmission of the signals corresponding
to the basebanded I and Q Doppler signals from implanted blood flow
meter 100 to a remote telemetry device 282 for further precessing.
In one embodiment, this further processing includes velocity determination
according to the well-known Doppler equation, illustrated in Equation
(1). ##EQU1##
In Equation (1): v is the blood flow velocity to be determined;
f.sub.d is the (basebanded) received Doppler shifted frequency reflected
from the blood flow, C is the speed of sound in the medium, e.g.
tissue; f.sub.c is the carrier frequency; and .theta. is the angle
formed by the velocity vector of the blood flow and the path along
which the illuminating ultrasonic energy is provided.
FIG. 2 is a block diagram illustrating one embodiment of mixer
205 in more detail. In FIG. 2 mixer 205 includes quadrature phase
splitter 300 first multiplier 305 and second multiplier 310. Splitter
300 receives, through node 115 the carrier signal at splitter input
315 and produces in response thereto a resulting in-phase carrier
signal at node 320 and a phase-shifted carrier signal at node 325
that is phase-shifted by 90 degrees with respect to the in-phase
carrier signal. The in-phase carrier signal at node 320 and the
phase-shifted carrier signal at node 325 are substantially quadruture
balanced, i.e. they are substantially matched in amplitude, and
have a phase difference which is very close to 90 degrees. The buffered
Doppler signal at node 215 is multiplied at first multiplier 305
by the in-phase carrier signal at node 320 to produce the I signal
at node 235. The buffered Doppler signal at node 215 is also multiplied
at second multiplier 310 by the phase shifted carrier signal at
node 325 to produce the Q signal at node 255.
FIG. 3 is a block diagram illustrating one embodiment of transducer
160 in more detail, in relation to blood vessel 105. In FIG. 3
transducer 160 includes ultrasound transmit transducer 330 and ultrasound
receive transducer 335. Transmit and receiver transducers 330 and
335 are preferably single piston piezoelectric transducers, comprised
of materials such as lead zirconate titanate (PZT) crystal or composite
materials. Other piezoelectric crystal, ceramic, or polymer, or
any other suitable transducer may also be used.
Transmit transducer 330 receives the electrical strobed amplified
ultrasonic-frequency signal at input 155 and provides, or launches,
continuous wave (CW) ultrasonic energy at transducer ultrasound
output 170 for illumination of blood vessel 105. Illumination of
blood vessel 105 results in a reflected Doppler-shifted ultrasound
signal at transducer ultrasound input 175 that is received by receive
transducer 335 and converted into an electrical received Doppler-shifted
signal at transducer electrical output 180. In FIG. 3 separate
transmit and receive transducers 330 and 335 are used for simultaneously
illuminating and receiving CW Doppler ultrasound. However, it is
understood that a single transducer could also be used for sequentially
illuminating and receiving pulsed Doppler ultrasound, as described
below.
FIG. 4 is a block diagram illustrating one embodiment of control
circuit 120 in more detail. In FIG. 4 control circuit 120 includes
sine wave to square wave converter 350 digital control logic 355
and strobing switch 360. Converter 350 receives the carrier signal
at node 115 and provides to digital control logic 355 a square wave
clock signal at node 365 which can be divided down to lower frequencies
if desire. Converter 350 is omitted if oscillator 110 is a square
wave, rather than a sine wave oscillator. Logic 355 provides a periodic
strobing control signal at node 370 also available at strobing
control signal output 371 to control the conductance of the carrier
signal at node 115 through strobing twitch 360 to control circuit
output 130. However, the periodic strobing control signal at node
370 could alternatively be provided at irregular intervals. A resulting
electrical strobed ultrasonic-frequency signal is provided through
node 145 for amplification by amplifier 140 and conversion into
ultrasound energy by transducer 160.
FIG. 5A is a voltage vs. time graph illustrating generally timing
in one embodiment in which the present invention is operated. FIG.
5A includes strobing control signal 370V at node 370 and the strobed
ultrasonic frequency signal 145V at node 145. A corresponding velocity
vs. time graph is illustrated in FIG. 5B, but with time illustrated
on a compressed time scale with respect to that in FIG. 5A. In FIG.
5A, strobing control signal 370 is a periodic control signal having
a corresponding strobing period, t.sub.strobe. The strobing period
is comprised of an active period, t.sub.on, and an idle period,
t.sub.off.
During the active period of the strobing control signal 370V, the
carrier frequency signal at node 115 is conducted to node 145 through
the strobing switch 360 as illustrated during the corresponding
portion of the strobed ultrasonic frequency signal 145V. During
the idle period of the strobing control signal 370V, the carrier
frequency signal at node 115 is isolated from node 145 by the strobing
switch 360 as illustrated during the corresponding portion of the
strobed ultrasonic frequency signal 145V. Blood vessel 105 is illuminated
during each active period of the strobing control signal 370V, as
illustrated in FIG. 5A. Velocity is determined near the end of each
active period of the strobing control signal 370V, such as at times
t.sub.1 t.sub.2 and t.sub.3 as illustrated in FIGS. 5A and 5B.
Blood velocity will vary depending on the size and physiological
location of the blood vessel 105 being measured. Blood velocity
will also vary as a function of time during the cardiac cycle, i.e.
during and between successive heartbeats. One embodiment of the
present invention uses a programmably adjustable strobing frequency,
which is the inverse of the strobing period. The strobing frequency
should be high enough to provide a representative estimated velocity
vs. time waveform both during the cardiac cycle and over many cardiac
cycles. For example, in most larger mammals, heart rate varies from
between 40 to 200 beats per minute. A strobing frequency of 50 Hz
respectively provides 75 and 15 estimated velocity data points for
each of these respective heart rates. For smaller mammals, such
as rats, heart rate may approach 400 beats per minute. Increasing
strobing frequency to 100 Hz would still allow 15 estimated velocity
data points for this case.
The particular strobing frequency may be selected to obtain the
desired time resolution of velocity estimates. The desired time
resolution of velocity estimates may in turn be selected to accommodate
the expected rate of change of blood flow velocity in the blood
vessel. The rate of change of the blood flow velocity is typically
higher for an arterial blood vessel 105 that is more proximal to
the heart than for an arterial blood vessel 105 that is more distal
from the heart or for a venal blood vessel 105. As set forth above,
t.sub.strobe will exceed t.sub.on. But the maximum value of t.sub.strobe
will depend on many factors, including whether an accurate reconstruction
of the velocity waveform is needed or whether the velocity estimates
are used only to determine blood flow, such that fewer estimates
per cardiac cycle may suffice.
In one embodiment, active period, t.sub.on, is minimized to minimize
average power consumption or to obtain other advantages, as described
below. However, the minimum active period is typically longer than
some combination of: a system bandwidth; a stabilization time; and
a mean-frequency estimation time.
The system bandwidth is defined as the inverse of the maximum expected
basebanded I and Q Doppler signal frequencies, which can be calculated
from the well-known Doppler equation for a particular blood velocity.
The stabilization time is the time required to power up and stabilize
certain electronic circuits which are powered down during the idle
period. The required stabilization time may be dominated by, for
example, the filter time-constants of first and second low pass
filters 245 and 265 if these filters were powered down during the
idle period. In another example, the required stabilization time
may be dominated by the charging of a power supply output capacitor
from which power is supplied to those electronic circuits that were
turned off during the idle period. Separate control signals may
be provided to individual electronic circuits to tailor the time
that the circuits are powered to meet their individual stabilization
requirements. For example, first and second low pass filters 245
and 265 may be turned on prior to providing the electrical signal
to drive transducer 160 to accommodate longer stabilization time
requirements of first and second low pass filters 245 and 265.
The mean frequency estimation time is determined by the number
of samples of the basebanded I or Q Doppler signals at respective
nodes 275 and 280 that must be acquired to accurately estimate the
blood velocity for a particular velocity estimate. The mean frequency
estimation time depends, in turn, on the particular mean frequency
estimation technique used. In one embodiment, sophisticated digital
signal processing techniques are used to extract a relatively accurate
mean frequency estimate from as few as 8 of the samples. In another
embodiment, zero-cross detection techniques are used to provide
a root mean square (rms) reading of mean frequency from more than
100 samples.
The present invention uses strobed ultrasonic energy, which advantageously
reduces its average power consumption. This is particularly important
when power is drawn from a fixed resource, such as a battery, which
is implanted in vivo together with the electronics of blood flow
meter 100 and cannot be easily replaced. In such situations, the
reduced average power consumption of the present invention is critical
for extending battery life of blood flow meter 100. The average
power consumption of the present invention is illustrated by Equation
(2). ##EQU2##
In Equation (2), P.sub.on is the power consumption during the active
period and P.sub.off is the power consumption during the idle period.
As explained below, most of the electronics of blood flow meter
100 are powered on during the active period, but only a subset of
these electronics are powered on during the idle period For this
reason, P.sub.on exceeds P.sub.off. Thus, as illustrated in Equation
(2), average power consumption is minimized by: reducing the duration
of the active period; and, increasing the strobing period; and,
decreasing both P.sub.on and P.sub.off, particularly P.sub.on.
FIG. 6 is a block diagram illustrating one embodiment of the present
invention in which only amplifier 140 and telemetry 285 are turned
off during the idle period. The strobing control signal at node
370 is electrically coupled to switchably control the conductances
between each of amplifier 140 and telemetry 285 blocks and their
respective power supplies. Transducer 160 typically does not draw
any bias current, but use of any transducer that does draw bias
current could similarly have its bias current switchably controlled
by strobing control signal 370. By leaving other blocks powered
during the idle period, stabilization time is reduced, as described
above. However, this embodiment does not minimize average power
consumption as much as other possible embodiments.
FIG. 7 is a block diagram illustrating another embodiment of the
present invention in which amplifier 140 receiver 190 mixer 205
fist and second low pass filters 245 and 265 and telemetry 285
are all turned off during the idle period. The strobing control
signal at node 370 is electrically coupled to switchably control,
either independently or in groups, the conductances between each
of amplifier 140 receiver 190 mixer 205 first and second low
pass filters 245 and 265 and telemetry 285 and their respective
power supplies. Since more components are powered down during the
idle period, this embodiment decreases average power consumption
further from that of FIG. 6 but stabilization time may be increased,
as explained above.
FIG. 8 is a block diagram illustrating another embodiment of the
present invention in which oscillator 110 amplifier 140 receiver
190 mixer 205 first and second low pass filters 245 and 265 and
telemetry 285 are all turned off during the idle period. The strobing
control signal at node 370 is electrically coupled to switchably
control, either independently or in groups, the conductances between
each of oscillator 10 amplifier 140 receiver 190 mixer 205 first
and second low pass filters 245 and 265 and telemetry 285 and their
respective power supplies. FIG. 8 uses a control circuit 400 which
is illustrated in more detail in FIG. 9. Since more components are
powered down during the idle period, this embodiment decreases average
power consumption further from that of FIGS. 6-7.
FIG. 9 is a block diagram illustrating in more detail the control
circuit 400 of FIG. 8. In FIG. 9 a separate timing generator 410
is provided for coupling a clock signal through node 365 to digital
control logic 355. As in the embodiments illustrated in FIGS. 1
and 6-7 at least a portion of the digital control logic remains
powered during the idle period in the embodiment illustrated in
FIGS. 8-9. In the embodiment illustrated in FIGS. 8-9 the timing
generator 410 also remains powered during the idle period. Timing
generator 410 is capable of being operated at a lower frequency
than the ultrasonic frequencies of oscillator 110. Use of timing
generator 410 allows the higher frequency oscillator 110 to be powered
down during the idle period. This results in further average power
savings in some implementations of the present invention.
Thus, the invention described above in FIGS. 1-9 provides a method
of estimating the velocity of blood flow in a blood vessel. At least
part of the measurement circuits are automatically activated only
during the time an estimate is being obtained. At least part of
the measurement circuits are deactivated during the time an estimate
is not being obtained. These steps are performed repeatedly to provide
a sequence of blood flow estimates forming a blood flow waveform
indicative of blood flow. More than one estimate is required to
obtain the blood flow waveform.
According to one embodiment of the present invention, ultrasonic
energy is repeatedly applied to the blood flow in the blood vessel,
either periodically or at irregular time intervals over a period
of time, such as during all or a portion of one or more cardiac
cycles. A portion of the applied energy is reflected from the blood
flow to produce a reflected ultrasonic energy signal. The reflected
ultrasonic energy is received for further processing from which
blood flow velocity is measured. Electronic circuits are powered
off or down between the repeated applications of ultrasonic energy,
thereby allowing increased levels of illumination while maintaining
or reducing average power consumption.
As described above, one embodiment of the present invention uses
strobed ultrasonic energy, which advantageously reduces its average
power consumption because portions of the present invention are
powered off between strobing instances. This advantage, or a portion
thereof, may be traded for improved signal-to-noise ratio (SNR),
which is also a desirable characteristic for accurate measurement
of blood flow velocity. For example, transducer 160 is capable of
providing higher level illumination of blood vessel 105 than in
a conventional system, because strobed ultrasonic energy is used,
i.e. the higher level illumination is limited to a shorter duration.
Since blood vessel 105 is illuminated at a higher level, more reflected
energy is available for detection, thereby improving the SNR.
Similar signal processing improvements are also available, for
example, by using higher supply currents for shorter durations in
those other blocks that are capable of being powered down during
the idle period, such as receiver 190 mixer 205 and first and
second low pass filters 245 and 265. These signal processing improvements
obtained from higher current levels for shorter durations include
better noise performance and higher bandwidth These improvements
provided by the present invention are particularly advantageous
for the receiver 190 and mixer 205 blocks, which require bandwidths
capable of accommodating a Doppler-shifted signal centered around
the 5-20 MHz carrier frequency. Thus, the strobed ultrasonic blood
flow velocity measurements of the present invention offer considerable
advantages in addition to reduced average power consumption.
Trading off the average power savings of the strobed CW Doppler
system of present invention for higher power during the active period
is further illustrated by way Example 1 comparing the present invention
to a conventional CW Doppler system
EXAMPLE 1
Conventional CW Doppler Strobed CW Doppler I.sub.avg = 2mA I.sub.avg
= 2mA t.sub.strobe = 20 ms (50 Hz strobing) t.sub.on = 2 ms t.sub.off
= 18 ms I.sub.idle = 500 .mu.A during t.sub.off I.sub.active = 15.5
mA during t.sub.on
Example 1 illustrates, for a 50 Hz strobing frequency and (t.sub.on
/t.sub.strobe)=10%, the strobed current can be as high as 15.5 mA
for an idle current of 500 .mu.A. Thus, in this example, the current
can be elevated by a factor of 7.75 in the strobed CW Doppler system
without increasing the average power consumption over a conventional
CW Doppler system.
FIG. 10 is a block diagram illustrating another embodiment of the
present invention in which an impedance matching network 450 has
been interposed between amplifier output 150 and transducer electrical
input 155. Network 450 includes passive impedance matching components
to maximize power transfer between amplifier 140 and transducer
160 at the carrier frequency, where amplifier 140 typically presents
an impedance that is unmatched to that of transducer 160.
The impedance matching network results in more efficient power
transfer at the output of network 450 at node 165B for the strobed
CW Doppler system of the present invention over a conventional CW
Doppler system, as illustrated in Example 2.
EXAMPLE 2
Conventional CW Doppler Strobed CW Doppler Z.sub.out = 2000.OMEGA.
at carrier frequency Z.sub.out = 400.OMEGA. at carrier frequency
Z.sub.tran = 20.OMEGA. at carrier frequency Z.sub.tran = 20.OMEGA.
at carrier frequency Z.sub.network = 100 to 1 matching Z.sub.network
= 20 to 1 matching I.sub.amp = 1 mA peak I.sub.amp = 5 mA peak during
active period V.sub.amp = 4V.sub.p-p continuous V.sub.amp = 4 V.sub.p-p
during active period P.sub.amp = 1 mW P.sub.amp = 10 mW P.sub.transducer
= 1 mW P.sub.transducer = 10 mW V.sub.transducer = 0.25 V.sub.p-p
V.sub.transducer = 0.89 V.sub.p-p
In Example 2: Z.sub.out is the output impedance of amplifier 140
at amplifier output 150 at the ultrasonic carrier frequency; Z.sub.tran
is the impedance of transducer 160 at the carrier frequency; Z.sub.network
is the impedance matching ratio of network 450; I.sub.amp is the
peak output current of amplifier 140; V.sub.amp is the peak-to-peak
output voltage of amplifier 140; P.sub.amp is the power output of
amplifier 140; P.sub.transducer is the power input of transducer
160; and, V.sub.transducer is the peak-to-peak input voltage of
transducer 160.
In Example 2 the conventional CW Doppler system is operated continuously,
and the strobed CW Doppler system is operated at a 10% duty cycle
(t.sub.on /t.sub.strobe) with the negligible current during the
idle period. As seen in Example 2 amplifier 140 and network 450
of the strobed Doppler system of FIG. 10 allow higher power output
from amplifier 140 and a higher input voltage of transducer 160.
This produces a higher level illumination, resulting in more reflected
ultrasonic energy, and thereby improving the SNR.
FIGS. 1-10 illustrate various embodiments of the present invention
in which the basebanded I and Q Doppler signals are telemetered
to other circuits for further processing to determine the blood
flow velocity estimate. In one embodiment, for example, the basebanded
I and Q Doppler signals are telemetered from an implanted portion
of the blood flow meter 100 to accompanying external circuits for
further processing. However, signal processing of the basebanded
I and Q Doppler signals can also be carried out within the implanted
blood flow meter 100.
FIG. 11 is a block diagram illustrating an embodiment of the present
invention in which a signal processor 470 is contained within the
implanted blood flow meter 100. In FIG. 11 signal processor 470
receives the basebanded I and Q Doppler signals at respective nodes
275 and 280 and produces a blood flow output signal or velocity
output signal representing the estimated blood flow velocity. The
velocity output signal is electrically coupled through node 475
to telemetry 285 where it is transmitted from the implanted blood
flow meter 100 to an external receiver.
FIG. 12 is a block diagram illustrating one embodiment of signal
processor 470 that is particularly useful in applications having
a single-ended power supply, such as a battery in the implantable
blood flow meter 100 of the present invention. In FIG. 12 signal
processor 470 contains a voltage reference 500 which provides a
stable output bias voltage at node 505 to a first input of each
of first and second amplifiers 510 and 520 and first and second
zero cross detectors 530 and 540. First and second amplifiers 510
and 520 provide gain, or provide both gain and level-shifting. First
and second amplifiers may also be used to provide bandpass filtering.
A second input of first amplifier 510 receives the basebanded I
Doppler signal at node 275. A second input of second amplifier 520
receives the basebanded Q Doppler signal at node 280.
First amplifier 510 provides a buffered basebanded I Doppler signal
at node 545 to a second input of first zero cross detector 530.
Second amplifier 520 provides a buffered basebanded Q Doppler signal
at node 550 to a second input of second zero cross detector 540.
First and second zero cross detectors 530 and 540 provide first
and second zero cross outputs at respective nodes 555 and 560. The
first and second zero cross outputs at respective nodes 555 and
560 each change logic state in response to the voltage of respective
buffered I and Q Doppler signals passing through the bias voltage
at node 505. Each of the resulting pulsatile voltages waveforms
at the first and second zero cross outputs is approximately 90 degrees
out of phase with the other, and is at the basebanded Doppler frequency.
Quadrature encoder 565 receives the first and second zero cross
outputs at respective nodes 555 and 560. The 90 degree phase difference
between the voltage waveforms at nodes 555 and 560 make it possible
to determine their phase relationship at each logic voltage transition
of these voltage waveforms at nodes 555 and 560. Quadrature encoder
565 contains logic circuitry for determining the phase relationship
between the first and second zero cross outputs at nodes 555 and
560 and does so at each voltage transition at each of nodes 555
and 560. In response to each such determination, quadrature encoder
565 provides a fixed-duration voltage pulse to only one of forward
node 570 or reverse node 575.
Differential frequency-to-voltage converter 580 receives voltage
pulses at each of the respective forward and reverse nodes 570 and
575 and provides a resulting blood flow output signal such as the
analog velocity output signal at node 475. In one embodiment, converter
580 provides charge integration of the fixed-duration voltage pulses
at each of the respective forward and reverse nodes 570 and 575
and provides the resulting blood flow output signal in response
thereto. The charge of the voltage pulses at the forward node 570
incrementally increases the velocity output signal at node 475
and the charge of the voltage pulses at the reverse node 575 incrementally
decreases the velocity output signal at node 475. Converter 580
could also be implemented as an up-down counter providing an output
count representative of the velocity output signal. Voltage pulses
received at forward node 570 increment the output count, and voltage
pulses received at reverse node 575 decrement the output count,
or vice versa.
Thus, signal processor 470 is capable of providing, using a single-ended
power supply, an analog velocity output signal at node 475 containing
both magnitude and directional information of blood flow velocity.
The analog velocity output signal at node 475 can be repeatedly
sampled to provide a sequence of blood flow estimates forming a
blood flow waveform indicative of blood flow. The analog velocity
output signal at node 475 or the samples derived therefrom can be
further processed and transmitted from the implanted blood flow
meter 100.
FIGS. 1-12 illustrate various bidirectional embodiments of the
present invention that are capable of determining the magnitude
and direction of blood flow velocity. If direction information is
not needed, a unidirectional embodiment of the present invention
could be used. In a unidirectional embodiment of the present invention,
one of the I or Q channels is omitted. In mixer 205 a quadrature
phase splitter 300 is omitted and only one of first and second multipliers
305 and 310 is needed. In signal processor 470 quadrature encoder
565 is replaced by a monostable oscillator (one-shot) providing
a fixed-duration pulse, and differential frequency-to-voltage converter
580 is replaced by a single-ended frequency-to-voltage converter.
The present invention has been described above with respect to
a particular embodiment of strobed ultrasonic Doppler blood flow
meter, i.e. a strobed continuous wave (CW) ultrasonic Doppler blood
flow meter, referred to as a strobed CW Doppler blood flow meter.
However, it is understood that the present invention is also broadly
applicable to any embodiment of a strobed ultrasonic Doppler blood
flow meter and its method of use.
For example, the invention encompasses the use of a strobed ultrasonic
pulsed Doppler blood flow meter, referred to as a strobed pulsed
Doppler blood flow meter. The strobed pulsed Doppler embodiment
also periodically illuminates a blood vessel by a transducer, but
each illumination comprises bursts of pulsatile (or pulse train)
ultrasonic-frequency energy. Each burst of ultrasonic-frequency
energy from a particular illumination is reflected, or backscattered,
from the blood flow and typically subsequently detected at the same
transducer. Samples of the resulting electrical signal, each corresponding
to a burst of pulsatile ultrasonic-frequency energy, are used to
estimate mean frequency. A resulting blood flow velocity estimate
is produced from the aggregation of mean frequency estimations within
a particular strobing.
FIG. 13 illustrates generally a comparison of the strobed ultrasonic
frequency signal waveforms used in each of the strobed CW and strobed
pulsed Doppler embodiments. In FIG. 13 the strobing control signal
370V illustrates generally the active and idle periods in relation
to the strobing period. The CW embodiment provides an ultrasonic
frequency signal 145V continuously over the entire active period
or at least some portion thereof. The strobed pulsed Doppler embodiment
provides a pulsed ultrasonic frequency signal 600 that typically
contains more than one burst of pulsatile ultrasonic-frequency energy
over the active period or at least some portion thereof.
In fact, as illustrated in FIG. 13 the type of ultrasonic energy
signal used is not essential to the invention. Thus, both of the
above-described ultrasonic blood flow meters have characteristics
that include: repeatedly illuminating the blood vessel with ultrasonic
energy during a cardiac cycle; repeatedly receiving during the cardiac
cycle an ultrasonic energy signal, which contains Doppler-shifted
frequencies corresponding to a blood flow velocity estimate, reflected
from the blood flow; and, processing the received ultrasonic energy
signal to obtain the blood flow velocity estimate from the Doppler-shifted
frequencies contained therein.
In both species of strobed ultrasonic blood flow meters, the ultrasonic
energy is strobed repeatedly throughout the cardiac cycle or other
period of interest, with a strobing frequency which is substantially
lower than the ultrasonic energy frequency. In one embodiment of
the present invention, each strobing instance corresponds to a resulting
blood flow velocity estimate.
The above-described embodiments describe a blood flow meter that
estimates blood flow velocity by strobed Doppler measurements of
backscattered ultrasonic energy. However, the strobed blood flow
meter according to the present invention also includes other techniques
of estimating blood flow velocity, including, but not limited to:
transit time measurements, electromagnetic flow measurements, thermal
dilution measurements, and laser Doppler measurements, each of which
is described further below.
FIG. 14 is a generalized schematic illustration of one embodiment
of a transit time measurement of blood flow velocity that is encompassed
by the present invention. First and second transducers 650 and 655
respectively, are configured for ultrasonic communication therebetween
via an acoustic reflector 660. A first ultrasonic impulse 665 is
launched from first transducer 650 reflected from reflector 660
and received at second transducer 655. A second ultrasonic impulse
670 is launched from second transducer 655 reflected from reflector
660 and received at first transducer 650.
FIG. 14 illustrates the case where first impulse 665 has a directional
component in the same direction as the blood flow in blood vessel
105 and second impulse 670 has a directional component opposite
the direction of blood flow in blood vessel 105. As a result, a
travel time of second impulse 670 from second transducer 655 to
first transducer 650 is longer than a travel time of first impulse
665 from first transducer 650 to second transducer 655. Blood flow
velocity is calculated from the difference in transit times of the
first and second impulses 665 and 670 respectively.
In this embodiment, the invention includes a control circuit 675
for providing a strobed ultrasonic frequency signal to each of respective
first and second amplifiers 680 and 685 through respective nodes
690 and 695. Control circuit 675 optionally provides power control
signals to respective first and second receivers 700 and 705 through
respective nodes 710 and 715. First and second amplifiers 680 and
685 respectively, provide an amplified strobed ultrasonic frequency
signal at respective nodes 720 and 725 to respective first and second
transducers 650 and 655 which provide the first and second impulses
665 and 670 in response thereto.
First and second transducers 650 and 655 also receive respective
second and first impulses 670 and 665 as described above, and provide
resulting electrical signals to respective first and second receivers
700 and 705 through respective nodes 730 and 735. First and second
receivers 700 and 705 respectively, provide buffered electrical
signals to processing circuit 740 through respective nodes 745 and
750. Processing circuit 740 calculates blood flow velocity from
the difference in transit times of the first and second impulses
665 and 670 respectively, and provides through node 755 a signal
containing blood flow velocity information to telemetry device 760
for transmission to a remote telemetry device. Control circuit 675
optionally provides a power control signal to processing circuit
740 through node 765 for reducing or removing power from processing
circuit 740 between transit time estimates of blood flow velocity.
As described above, control circuit 675 may also optionally provide
a power control signal to telemetry device 760 to reduce or remove
power from telemetry device 760 when it is not transmitting a transit
time estimate of blood flow velocity.
FIG. 15 illustrates an end view of the configuration of FIG. 14.
In FIG. 15 first and second transducers 650 and 655 respectively,
and reflector 660 are arranged such that first and second impulses
665 and 670 respectively, each provide an insonification area 770
that includes the entire area of blood vessel 105 such that an
average estimate of blood flow over the area of blood vessel 105
is provided. The transit time estimate of blood flow velocity may
also be improved by averaging multiple transit time measurements
to provide a single estimate of blood flow velocity. In such an
embodiment, control circuit 675 reduces or removes power from other
circuits between each series of transit time measurements used to
provide a blood flow velocity estimate. A sequence of blood flow
estimates forms a waveform representative of blood flow over a period
of time.
The present invention also includes the use of electromagnetic
flow techniques to estimate blood flow velocity. In one embodiment
of this technique, first and second electrodes are disposed across
an interposed blood vessel such that the blood flow is in a direction
that is substantially orthogonal to a vector between the first and
second electrodes. A permanent magnet or electromagnet is used to
create a magnetic field through the blood vessel in a direction
that is substantially orthogonal to both the direction of blood
flow and the vector between the first and second electrodes. As
a result, ionized particles within the blood flow are deflected
toward one of the first and second electrodes, resulting in a voltage
difference therebetween that is proportional to the blood flow velocity.
The invention uses the above-described strobing technique to reduce
or remove power between blood flow estimates to circuits within
the blood flow meter, such as to the electromagnet, if any, or to
sensing and processing circuits that detect the voltage difference
between the first and second electrodes, or to telemetry circuits
that transmit electromagnetic flow estimates of blood flow velocity
to a remote telemetry device.
The present invention also includes the use of thermal dilution
techniques to estimate blood flow. In one embodiment of this technique,
a heater is used to pulsedly heat the blood, and the heated blood
pulse is detected by a temperature sensor located at a known distance
from the point of heating in the direction of the blood flow. Volumetric
blood flow is calculated from the time between the heating of the
blood pulse and the detection of the blood pulse. Several heated
blood pulses are typically introduced and detected to produce a
more accurate blood flow estimate.
In another embodiment of this technique, a single thermistor is
used for both heating and detection. A heated thermistor is introduced
into the blood vessel such that it is in thermal contact with the
blood flow, and cooling of the thermistor is effected by the blood
flow. Blood flow at a higher velocity cools the thermistor at a
higher rate than blood flow at a lower velocity. The energy delivered
to the thermistor to maintain the thermistor at a constant temperature
is proportional to blood flow velocity. Alternatively, the thermistor
can be heated to a known temperature, and the time required to cool
the thermistor to a second, lower temperature will be inversely
proportional to blood flow.
According to the present invention, measuring circuits in the above-described
thermal dilution embodiments are automatically activated only during
estimation of blood flow, and are powered down or off between estimates
of blood flow. A resulting volumetric blood flow vs. time waveform
constructed from the sequence of blood flow estimates is thereby
obtained at a reduced power consumption by application of the strobing
technique of the present invention.
The present invention also includes the use of laser Doppler techniques
to estimate blood flow. The blood flow is illuminated with a coherent
monochromatic light source signal. A resulting backscattered Doppler-shifted
light signal is received at an optical detector, and demodulated
such as by mixing with the monochromatic light source signal. Blood
flow velocity is estimated from a resulting basebanded Doppler-shifted
frequency of the received light signal. According to the present
invention, measuring circuits, optionally including the monochromatic
light source, are automatically activated only during estimation
of the blood flow velocity. These measuring circuits are deactivated,
i.e. powered down or off between estimates of blood flow velocity.
A resulting velocity vs. time waveform constructed from the sequence
of blood flow velocity is thereby obtained at a reduced power consumption
by application of the strobing technique of the present invention.
Thus, the present invention provides an strobed blood flow meter,
such as an implantable strobed ultrasonic Doppler blood flow meter,
having reduced average power consumption, which is advantageous
for reducing battery size, improving signal-to-noise ratio, and
extending battery life.
It is to be understood that the above description is intended to
be illustrative, and not restrictive. Many other embodiments will
be apparent to those of skill in the art upon reviewing the above
description. The scope of the invention should, therefore, be determined
with reference to the appended claims, along with the fill scope
of equivalents to which such claims are entitled. |